The combination of an optical fiber's low mass, low moment of inertia, and light damping results in large amplitudes or large angular deflections at its tip when excited into resonance. Small movements of the actuator at the base of an optical fiber (base excitation) or weak forces produced by the actuator either along the length of the fiber or at its tip, efficiently result in large amplitudes or large angular deflections at the fiber's tip. When driven to move at resonance or near resonance, an optical fiber scanner can be used for image display and acquisition, as well as the basis of several fiber optic sensors, e.g. media density, temperature, or proximity to a surface (atomic force microscopy).
Most optical scanning applications use a moving mirror, either rotating or oscillating. A laser beam is often projected onto the moving mirror to scan the beam across a specified linear or two-dimensional (2D) (raster) pattern at a frequency that is sufficient for the particular application. For optical displays, the field of view (FOV) is determined by the scanning amplitude and the particular optical design. There is a minimum frequency (rate) at which scanning displays need to be refreshed, which is determined by the human perception of flicker from a scanned display. For ubiquitous raster scanning displays, such as cathode ray tubes (CRTs) used in televisions and computer monitors, the display refresh rate is typically 30 to 60 Hz. Although a CRT employs an electron beam for scanning an electro-optical display screen, the same requirements for scan frequency and amplitude (that determine the FOV) generally apply for all types of scanning displays. Thus, for a super video graphics array (SVGA) display having a CRT resolution of 800×600 pixels, the minimum horizontal scan rates are 40 kHz for unidirectional and 18 kHz for bi-directional scanning.
Combining both high resolution (>100,000 pixels) and wide FOV (>30°) in a single display is a difficult technical challenge, limiting the application of optical scanning for small size, low cost optical scanners that have both high resolution and wide FOV. To date, a mirror-based resonant scanner fabricated as a micro-electromechanical systems (MEMS) device has yet to be demonstrated as a viable method for manufacturing low cost optical scanners for visual displays of wide FOV and at video scan rates.
There is a growing market for micro-optical displays as well as small optical sensors, optical switches, and scanning image acquisition systems. For example, a low cost micro-optical scanner is essential for spectacle-mounted, retinal light scanning displays and micro-displays that may be embedded in future cellular telephones. Moreover, there is a commercial need for low cost, large-scale (panoramic) optical displays, because larger CRT displays are uneconomical in energy and space. There is also a growing market for optical sensing and switching, especially in conjunction with fiber-optic sensing and communication applications. Finally, the lack of low cost micro-optical scanners with a wide FOV has been the most significant barrier for reducing the size of scanning image acquisition systems for use in surveillance, industrial inspection and repair, machine and robotic vision systems, micro-barcode scanners, and minimally-invasive medical imaging, e.g., a flexible single fiber scanning endoscope (SFSE).
To address some of the problems noted above with mirror-based scanners, optical fiber scanners have been developed that are relatively compact and usable for either image acquisition or image display. The scanning optical fiber is preferably actuated to move either in one dimension or in two dimensions using, for example, piezoelectric bimorph or tube actuators. By tapering a distal end of the optical fiber to a relatively small size, large FOVs have been obtained as the optical fiber is driven to move relatively to a surface. When used for image acquisition, one or more laser light sources coupled to the proximal end of the optical fiber provide light that is emitted from the distal tip as it scans a surface. The actuator(s) can cause the distal tip to scan the surface in a linear motion or a space-filling motion, such as a raster pattern, a spiral pattern, a propeller pattern, various Lissajous scanning patterns, and in other desired patterns. One or more photodetectors disposed adjacent to the distal tip (or elsewhere, if other optical fibers convey the reflected light to remote photo detectors) respond to light reflected from the surface, producing a signal that can be processed or employed to produce an image of the surface being scanned. When used to display an image, a modulated light source responds to an input signal producing light that is then emitted by the distal tip of the scanning optical fiber as it scans either an adjacent surface, or a user's eye (i.e., retina). The modulated light emitted is applied to create a pixilated array of light spots on the surface or on the retina of a user's eye, forming an image. However, these and various other applications of a scanning optical fiber require control of the actuators employed to cause the distal tip of the optical fiber to move in the desired scanning pattern. Various problems must be addressed by the control scheme employed for this purpose.
For example, although spiral pattern scans can be implemented efficiently with a very compact scanning optical fiber, it has been found that a spiral pattern of light emitted by a scanning optical fiber is subject to distortion that adversely affects the spiral pattern. In the quest to increase the performance of a SFSE and scanning optical fibers used in other applications, simulations have been run to discover the source of the distortions or breakdown of the spiral scan pattern. These simulations, which are based upon a nonlinear model of a piezo-tube driven base excited resonating fiber, show that the interactions of low damping, transient response after discontinuities, and nonlinear amplitude and phase responses are the major sources of distortion.
Because a resonant amplification occurs only in a small frequency range around the resonant frequency of an optical fiber, the optical fiber acts as a band-pass filter (amplifier) between an actuator input and the resulting motion of the optical fiber (i.e., its scanning motion). Other that at the fundamental frequency, many of the frequency components of complex scan patterns (e.g., square or triangle waves) are not sufficiently amplified to provide the corresponding complex motion. As a result, the optical fiber scanners are usually used to produce nearly sinusoidal scans, typically at a constant amplitude and phase, with respect to the drive signal applied as an actuator input.
The amplitude and phase response of an optical fiber scan may vary greatly between scanners and within the same scanner over time, due to differences or changes in the scanner resonant properties. Between scanners, differences in resonant properties are due to manufacturing variability either in the length of the fiber, quality of actuator/fiber coupling, or actuator efficiency. A single scanner's resonant properties may change due to environmental effects (temperature changes) or aging (fiber cracking, actuator/fiber coupling deterioration). Generally, it is preferable to achieve a consistent behavior between scanners, and within the same scanner over time.
The low mass and light damping of the fiber, while essential for large resonant amplification, also allow disturbances to persist for long periods of time. Also, optical fibers undergoing large deflections exhibit nonlinear behavior. This behavior includes amplitude and phase shifts in the output that are dependent on the amplitude of the input, cross-coupling of the optical fiber vibration axes, and possible bi-stable output amplitude. This nonlinear behavior produces undesirable scan distortion or inconsistencies.
Various control schemes can be employed in controlling the scanning motion of an optical fiber. In regard to the spiral scan problem noted above, it would be desirable to remove the distortion and make the scan pattern robust to scanner variations. An appropriate controller should be capable of asymptotically tracking the spiral scan pattern with minimal error and maximum robustness. To achieve acceptable results in an appropriate control for achieving a spiral scan and other desired scanning patterns, an appropriate control approach must be developed.